A new technique is proposed to measure the concentration of leukocyte precursor cells in the bone marrow and blood of patients during antineoplastic chemotherapy. The Agar Diffusion Chamber Precursor Cell (ADCPC) forms discrete colonies and clusters in Millipore chambers implanted in irradiated mice and is not dependent upon a standardized source of colony-stimulating activity. It is planned to assay serially ADCPC concentration in the blood and marrow of patients treated with intermittent chemotherapy and to determine the prognostic value of the assay in adult acute leukemia. It is proposed to develop the assay of ADCPC in dog blood and marrow and to use this animal model for evaluation and development of chemotherapeutic regimens of potential value in man.